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Abstract:

The invention refers to a device and a tool for cold expansion of
fastener holes. The advantages of the device and tool are in the
possibility to control the degree of cold expansion. The device (61)
comprises a hydraulic cylinder (27) in which a piston (26) with a piston
rod (25) is placed, to whose end a mandrel (1) of a tool (60) is fixed.
To flange (9) a threaded bush (7) of the tool (60) is immovably fixed,
such that in an axial blind hole (55) of the piston rod (25), a linear
displacement sensor (34), is positioned, immovably fixed to the hydraulic
cylinder (27). A magnet (33) is designed for activating the linear
displacement sensor (34), which is connected to the control block (38).

Claims:

1. A device for cold expansion of fastener holes by means of plastic
deformation comprising a hydraulic cylinder in which a piston with a
piston rod is placed to whose end a mandrel of a tool is fixed for
performing the expansion, wherein a threaded bush (7) of the tool (60) is
immovably fixed to the body (9) of the hydraulic cylinder (27) of the
device (61) and, such that in an axial blind hole (55) of the piston rod
(25) a linear displacement sensor (34) is placed and fixed immovably to
the hydraulic cylinder (27), and in an axial hole (58) of the piston (26)
a magnet (33) is fixed immovably designed to activate the linear
displacement sensor (34) which is connected to a control block (38),
electrically connected to a pressure sensor (37) and to electromagnets
(42) and (41) of hydraulic control valve (43), which is hydraulically
connected to two `throttle controllable--non-return valve` blocks (39 and
40), designed respectively for both directions of the working fluid
motion.

2. A tool for cold expansion of fastener holes comprising a mandrel with
a taper working part, such that between the mandrel and the hole to be
worked is placed a longitudinally split into three, four or more parts
cylindrical sleeve with a conical hole at one end whose cone angle is
equal to the angle of the mandrel working conical part, wherein the
cylindrical sleeve (4) is entirely split from end to end into separate
touching sideways one another without a clearance, segments (6), which
are fixed without a clearance at the other end in a cylindrical hole (14)
of the threaded bush (7) connected to the device (61) and along the outer
cylindrical surface (8) of the mandrel (1), such that the threaded bush
(7) is attached immovably to a driving device, fixing the segments (6) in
axial direction as well, and each segment (6) gets into contact with the
conical surface (2) of the mandrel (1) along only one generatrix (16),
lying in the symmetry plane (17) of the respective segment for each
reciprocal position of the mandrel (1) and segments (6).

3. A tool for cold expansion of fastener holes according to claim 2,
wherein around the outer cylindrical surfaces (19) of the segments (6),
at a distance of up to 10 mm from their working surfaces (10), in contact
with the hole (3) to be cold expanded, an elastic element (21) is
attached to return the segments (6) to their original position.

4. A tool for cold expansion of fastener holes according to claim 2,
wherein the segments (6) are positioned in the cylindrical hole (14) of
the threaded bush (7) with a possible radial displacement, such that
between the segments (6) and the threaded bush (7) an elastic element
(18) is placed.

5. A tool for cold expansion of fastener holes according to claims 2 and
4, wherein the conical hole (5) in the split sleeve (4) and the conical
surface (15) of the working part (2) of the mandrel (1) expand in the
direction of the split sleeve (4) end which is in contact with the hole
(3) to be deformed plastically.

6. A tool for cold expansion of fastener holes according to claims 2, 4
and 5 wherein the homonymic rotational outer surfaces (19) with radii
Rie of the segments (6) form a respective continuous outer
surface which is not smooth due to longitudinal edges formed by the side
contacts between the segments (6), such that the radii of the circles
circumscribed around the obtained continuous but not smooth outer
surfaces are rie, whereat: R i = r i ( 1 + k 2 sin
π n + 2 k tg π n ) 1 2 , where
k = δ 2 r i , ##EQU00004## δ is the width of
the tool with which the cylindrical sleeve (4) is split longitudinally,
and n is the number of segments (6).

7. A tool for cold expansion of fastener holes according to claims 2, 4
and 5, wherein the homonymic rotational outer surfaces (19) with radii
Rie of the segments (6) form a respective continuous outer
surface which is smooth.

8. A tool for cold expansion of fastener holes according to claims 2, 4,
5 and 6, wherein the homonymic rotational inner surfaces (20) with radii
Rii of the segments (6) form a respective continuous inner
surface which is not smooth due to longitudinal edges formed by the side
contacts between the segments (6), such that the radii of the inscribed
circles in the obtained continuous but not smooth inner surfaces are
rii, whereat : R i i = r i + δ 2 sin π n
, ##EQU00005## where δ is the width of the tool with which
the cylindrical sleeve (4) is split longitudinally, and n is the number
of segments (6).

9. A tool for cold expansion of fastener holes according to claims 2, 4,
5 and 7, wherein the homonymic rotational inner surfaces (20) with radii
Rii of the segments (6) form a respective continuous inner
surface which is smooth.

10. A tool for cold expansion of fastener holes according to claims 2, 4
and 5, wherein the cylindrical surfaces (10) of the segments (6), having
a contact with the hole (3), to be cold expanded have radii R,
determined by the formula: R = d 0 2 ( 1 + D C
E 100 ) , ##EQU00006## where d0 is the diameter of the hole
previously drilled, DCE is the degree of cold expansion in percentage.

Description:

FIELD OF THE INVENTION

[0001] The field of the invention relates to a device and a tool for cold
expansion of fastener holes by means of plastic deformation and can be
applied in working of holes in structural elements subjected subsequently
to cyclic external loads, which are made of tough-plastic materials and
of anisotropic aluminum/carbon fiber reinforced plastics.

BACKGROUND OF THE INVENTION

[0002] From U.S. Pat. No. 4,665,732 a device is known for cold expansion
of holes by means of a tool comprising a longitudinally split hollow
mandrel. A round pin is introduced into the axial hole of the mandrel.
The device comprises a hydraulic cylinder in which a first piston with a
piston rod is placed to whose end the longitudinal split hollow mandrel
of the tool is fixed. A support in contact with the work piece to be
worked during pulling the hollow mandrel through the hole, is fixed
immovably. In an axial hole worked in the first piston and its piston
rod, a second piston rod is inserted whose end is connected to the round
pin which makes the mandrel rigid. A second piston connected to the
second piston rod is placed in a second cylinder coaxial to the first one
and connected to it. The device has a power hydraulic system driven and
controlled by means of compressed air.

[0003] When working fastener holes using the known device and tool the
degree of cold expansion of the hole depends only on the diameter
dimensions of the mandrel and pin as well as on the diameter of the
previously drilled hole. To guarantee the degree of cold expansion
specified with a certain tolerance, it is necessary to control by
geometric criterion by means of gauges both the diameter of the
previously drilled hole and the mandrel working part.

[0004] Furthermore, working is carried out sequentially, expansion is
performed layer by layer along the hole axis, whereupon a deformation
wave passes causing at every point of the hole surface alternating
stresses whose absolute value exceeds significantly the yield limit of
the material. Due to a support placed against the movement of the
stiffened mandrel, an axial force flow passes through the work piece
being worked and as a result an asymmetric axial gradient of the
generated zone of compression residual stresses around the hole is
obtained. At the hole entrance residual stresses with minimum absolute
value occur. Furthermore, at the entrance and exit, around the hole edge,
significant axial deformations appear known as "surface upset"

[0005] As a result of all this, when employing the known device and tool
for cold extension, along the axis of the hole to be worked, a
non-uniform and asymmetric zone of compression residual stresses arises,
which is a prerequisite for fatigue cracks growth, initiating from the
entrance face with residual stresses with minimum absolute value. The
axial deformation along the hole edges are a prerequisite for wear and
growth of microcracks in consequence of the detrimental fretting effect.
As a result, cyclically or dynamically loaded structural elements will
have shorter fatigue life and respectively less reliability. In case of
static loads, the resistance of the elements to crack appearance will be
lowered.

[0006] From BG Patent No 66052 a tool for cold expansion of holes is
known, comprising a mandrel with a conical working part. Between the
mandrel and the hole to be worked is positioned a longitudinally split
into three, four or more portions cylindrical sleeve with a conical hole
at one end which has a cone angle α equal to the angle of the
working conical part of the mandrel. The longitudinally split
cylindrically shaped sleeve comprises an immovably fixed non-split part
in its other end which is 1/5 of the total length of the sleeve. As a
result of the relative axial displacement between the mandrel and sleeve,
its split part enhances its radial dimensions, expanding in this way the
hole being worked, on the account of inserting the conical working part
of the mandrel into the conical hole of the sleeve.

[0007] When expanding fastener holes with the known tool, there is an edge
contact between the mandrel conical surface and the conical surfaces of
the working parts of the sleeve during their relative motion along their
common axis because circular cross sections with a larger radius get into
contact with the cross sections of the working parts of the sleeve having
a smaller radius. In this way, in the process of working the hole, the
mandrel conical surface is in contact with longitudinally oriented edges
of the working parts of the split sleeve. As a result, the transfer
function between the axial displacement of the mandrel and the radial
displacement of the working parts of the split sleeve is non-linear, and
therefore practically it is not possible to control a precisely preset
degree of cold expansion of the hole to be worked. At the same time, this
edge contact leads to useless increase in the axial force applied to the
mandrel, required for working the hole. Furthermore, the edge contact
parameters vary at each instant of the mandrel motion and hence the
driving device is subjected to undesired pulsations. At the initial
moment of the cold expansion process when the working parts of the sleeve
are positioned in the hole to be worked, between every two working parts
there is a clearance equal to the thickness of the tool with which the
sleeve was split. In the process of cold expansion this clearance
increases. As a result, in the cross section of the hole, the zone of
compression residual stresses, though it has a pole of symmetry, it is
non-homogeneous. Furthermore, the requirement that the outer cylindrical
surface of each working part should have a radius larger than the radius
of the previously drilled hole that is to be expanded, causes
manufacturing difficulties when making the sleeve, due to the fact that
it is not entirely split.

TECHNICAL DESCRIPTION OF THE INVENTION

[0008] The object of this invention is to develop a device and a tool for
cold expansion of fastener holes, making possible the realization of
various degrees of cold expansion with one tool which has an improved
manufacturability and herewith for running the process a smaller axial
force is required and the compression residual stress zone is practically
uniform not only in axial but also in circumferential direction.

[0009] The object has been solved by developing a device for cold
expansion of fastener holes by means of plastic deformation comprising a
hydraulic cylinder in which a piston with a piston rod is placed to whose
end a mandrel is fixed for performing the expansion. According to the
invention, a threaded bush of the tool is immovably fixed to the body of
the hydraulic cylinder of the device. In an axial blind hole in the
piston rod, a linear displacement sensor is placed and fixed immovably to
the hydraulic cylinder, and in an axial hole in the piston a magnet is
fixed immovably designed to activate the linear displacement sensor. The
latter is connected to a control block, electrically connected to a
pressure sensor and to electromagnets of hydraulic control valve which is
hydraulically connected to two `throttle controllable-non-return valve`
blocks, designed respectively for both directions of the working fluid
motion.

[0010] The object has also been solved by a tool for cold expansion of
fastener holes comprising a mandrel with a taper working part. Between
the mandrel and the hole to be worked is placed a longitudinally split
into three, four or more parts cylindrical sleeve with a conical hole at
one end, whose cone angle is equal to the angle of the mandrel working
conical part. According to the invention, the cylindrical sleeve is
entirely split from end to end into separate, touching sideways one
another without a clearance, segments which are fixed without a clearance
at the other end in a cylindrical hole of the threaded bush connected to
the device and along the outer cylindrical surface of the mandrel. The
threaded bush is attached immovably to a driving device fixing the
segments in axial direction. Each segment is in contact with the conical
surface of the mandrel along only one generatrix lying in the symmetry
plane of the respective segment for each reciprocal position of the
mandrel and segments.

[0011] Preferably, around the outer cylindrical surfaces of the segments,
at a distance of up to 10 mm from their working surfaces contacting the
hole to be cold expanded, an elastic element is attached for returning
the segments to their original position.

[0012] Preferably also, the segments to be positioned in the cylindrical
hole of the threaded bush with a possible radial displacement such that
between the segments and the threaded bush an elastic element to be
placed.

[0013] The conical hole in the split sleeve and the conical surface in the
mandrel working part expand in the direction of the split sleeve end
which contacts the hole to be plastically deformed.

[0014] In a variant of the embodiment, the homonymic rotational outer
surfaces with radii Rie of the segments form a respective
continuous outer surface which is not smooth due to longitudinal edges
formed by the side contacts between the segments such that the radii of
the circles circumscribed around the obtained continuous but not smooth
outer surfaces are rie, whereat:

δ is the width of the tool with which the cylindrical sleeve is
split longitudinally, and n is the number of segments.

[0015] It is possible, in another variant of the embodiment, the homonymic
rotational outer surfaces with radii Rie of the segments to
form a respective continuous outer surface which is smooth.

[0016] Respectively, in the first variant of the embodiment, the homonymic
rotational inner surfaces with radii Rie of the segments, form
a respective continuous inner surface which is not smooth due to
longitudinal edges formed by the side contacts between the segments and
the radii of the inscribed circles in the obtained continuous but not
smooth inner surfaces are rii, whereat:

R i i = r i + δ 2 sin π n , ##EQU00002##

where δ is the width of the tool with which the cylindrical sleeve
is split longitudinally, and n is the number of segments.

[0017] In the second variant of the embodiment, the homonymic rotational
inner surfaces with radii Rii of the segments form a respective
continuous inner surface which is smooth.

[0018] The cylindrical surfaces of the segments having a contact with the
hole to be cold expanded, have radii R, determined by the formula:

R = d 0 2 ( 1 + D C E 100 ) ,
##EQU00003##

where d0 is the diameter of the hole previously drilled, DCE is the
degree of cold extension in percentage.

[0019] The advantages of the device are in the possibility to control the
degree of cold extension when the device is supplied with a tool having
specified dimensions according to the invention. The process of cold
extension carried out with the device is not dimensional, since with one
and the same tool different degrees of cold extension are realized
depending on the axial stroke of the mandrel after reaching the set
minimum pressure on the piston, respectively, after reaching a tight
contact between the conical part of the mandrel, segments and hole. In
this way a relatively wider tolerance of the diameter of the previously
drilled hole is allowed and it is not necessary a check with a gauge as
it is typical of dimensional processes.

[0020] Another advantage is that an axial force flow does not pass through
the work piece being worked because the flow closes itself passing
through the device and tool and a possibility is provided for creating a
practically uniformly distributed in axial direction zone of compressive
residual stresses having a minimal and symmetrical gradient around the
worked fastener holes in structural elements. As a result, the cyclically
loaded structural elements will have longer fatigue lifetimes.

[0021] Owing to the purely radial effect of segments on the hole surface
and lack of friction during sliding in axial direction, the axial
deformations along its two edges--surface upset will be minimum which
will minimize the possibility the detrimental fretting effect to appear.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] These and further features of the present invention will be
apparent with reference to the following description and drawings,
wherein:

[0023] FIG. 1 is a longitudinal section of the device and tool.

[0024] FIG. 2 is alongitudinal section through the tool with bending
loaded segments.

[0029] FIG. 7 is a longitudinal section through a tool with radially
moving segments.

[0030] FIG. 8 is a longitudinal section through another embodiment of the
tool.

[0031] FIG. 9 is a diagram of the driving electro-hydraulic system
connected to the device.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

[0032] According to a preferred embodiment of the invention the device 61
for cold expansion of fastener holes comprises (FIG. 1) a hydraulic
cylinder 27, in which a piston 26 is placed with a seal 31, connected to
a piston rod 25, in whose hole 24 by means of thread 23 a mandrel 1 of
the tool 60 is fixed. To the hydraulic cylinder 27 a flange 9 is fixed to
which a threaded bush 7 of the tool 60 is fixed immovably. In an axial
blind hole 55 of the piston rod 25 a linear displacement sensor 34 is
positioned which by means of threaded joint 56 is fixed immovably to
hydraulic cylinder 27. A control block 38 (FIG. 9) is comprised,
corresponding with the linear displacement sensor 34, with the pressure
sensor 37 and with electromagnets 41 and 42 of hydraulic control valve
43, which is fed with working fluid by means of a hydraulic pump 46. The
hydraulic control valve 43 corresponds in sequence with two `throttle
controllable-non-return valve` blocks 39 and 40, depending on the
direction of working fluid motion. A safety valve 45 and a pressure gauge
44 are included to control the pressure of the working fluid. The working
fluid is stored in a tank 48 and it is filtered by means of a filter 47.

[0033] Between the flange 9 and the hydraulic cylinder 27 a shutter 28 is
placed coaxially with a sealing ring 29. The shutter 28 is immovably
fixed to the hydraulic cylinder 27 by means of a threaded connection 57.
In axial hole 58 of the piston 26, without an axial clearance are
positioned a spacer bush 32 and a magnet 33, needed for activating the
linear displacement sensor 34. The hydraulic cylinder 27 is placed in a
plastic casing 30, having a hollow handle 35 with a start button 36. At
the end of the handle 35 pipe unions 53 and 54 are fixed. In the
hydraulic cylinder 27 radial holes 49 and 52 are made, through which the
working fluid passing through the pipe unions 53 and 54, enters
respectively piston chamber 50 and rod chamber 51 of the hydraulic
cylinder 27.

[0034] The tool 60 for cold expansion of holes (FIG. 2) comprises a
mandrel 1 with a conical working part 2, such that between the mandrel 1
and the hole 3 to be worked in the work piece 12, having front face 59, a
cylindrical sleeve 4 is positioned. It is longitudinally split into
three, four or more parts called segments 6, and has a conical hole 5 in
one end whose cone angle is equal to angle α of the working conical
part 2 of the mandrel 1. The cylindrical sleeve 4 is entirely split from
end to end into separate segments 6, and then they are radially moved to
the common axis 13 of the mandrel 1 and the sleeve 4 so that to touch
each other without a clearance. At their other end the segments 6 are
placed without a clearance in a cylindrical hole 14 of the threaded bush
7 and along outer cylindrical surface 8 of the mandrel 1, which is in
contact with the inner cylindrical surface 20 of the segments 6. The
threaded bush 7 is fixed immovably to the flange 9 of device 61 for cold
extension, fixing segments 6 in axial direction as well. Each segment 6
gets into contact with conical surface 15 of the conical working part 2
of the mandrel 1 along only one generatrix 16, lying in the symmetry
plane 17 of the respective segment 6 for each reciprocal position of the
mandrel 1 and segments 6. To facilitate mounting, chamfers 22 have been
made on segments 6.

[0035] Around outer cylindrical surfaces 19 of segments 6, at a distance
of up to 10 mm from their working surfaces 10, in contact with the hole
to be cold expanded, an elastic element 21 is attached designed for
returning the segments 6 to their original state of sideway touching
after their radial displacement. In this way the introduction of segments
6 into the hole 3 to be worked is facilitated. The introduction of
segments 6 is also facilitated by chamfers 11, made on their working
surfaces 10.

[0036] Before the cylindrical sleeve 4 was split along its axis, it had
had radii of its outer and inner rotational surfaces, respectively
Rie and Rii. After splitting the cylindrical sleeve 4
and radial displacement of the resulting segments 6 to the axis 13 up to
their sideway touching without a clearance, the homonymic rotational
surfaces of the segments 6 form the respective continuous surface which
is not smooth due to longitudinal edges formed by the side contacts
between the segments 6. The radii of the circumscribed circles around the
obtained continuous but not smooth outer surfaces are rie, and
the radii of the inscribed circles around the obtained continuous but not
smooth inner surfaces are rii. The two groups of radii, on one
hand Rie and Rii, and on the other hand rie
and rii, are in correlation. If the radii rii and
rii are known, as well as the number n of the segments and the
width δ of the tool with which the cylindrical sleeve 4 is
longitudinally split, the diameter dimensions, respectively the radii
Rie and Rii, of the cylindrical sleeve 4 with which
the latter to be made, can be calculated.

[0037] With a view to make a hole with minimum deviation from the correct
geometrical shape after the cold expansion, the working cylindrical
surfaces 10 (FIG. 6) of the segments 6, which are in contact with the
hole 3 to be worked, can be worked with radius R, which depends on the
diameter d0 of the previously drilled hole and on the degree of cold
extension in percentage DCE. For instance, if d0=4 mm, DCE=4%, it
follows for R:R=3.12 mm.

[0038] In one embodiment of the tool 60 (FIG. 8), the segments 6 are
placed in the cylindrical hole 14 of the threaded bush 7 with a
possibility for radial displacement such that between the segments 6 and
threaded bush 7 an elastic element 18 is placed.

[0039] In another embodiment of the tool 60 (FIG. 7), the conical hole 5
in the split sleeve 4 and the conical surface 15 of the working part 2 of
the mandrel 1 expand in the direction of the end of the split sleeve 4.
This end is in contact with the hole 3 which is to be plastically
deformed.

[0040] Cold expansion of fastener holes by means of the device 61 and tool
60 is carried out in the following way: by means of a handle 35 of the
device 61 the operator introduces the segments 6 into the previously
drilled hole 3 of work piece 12, which is facilitated by the chamfers 11
and elastic element 21. The end of the working surfaces 10 of the
segments 6 coincides with the hole edge from the front face 59 of work
piece 12, as it is shown in FIG. 2. There is a clearance between the
segments 6 and the hole 3.

[0041] In advance, for the whole class of holes to be worked with the
mounted in device 61 tool 60, in the control block 38 minimum pressure
pmin and length ha of the axial stroke of the mandrel 1 are
preset with a view of conducting two successive checks, respectively a
check for reaching the initial minimum pressure pmin, which
guarantees hard contact between the conical working part 2 of the mandrel
1, the segments 6 and the hole 3 to be worked and a second check for the
following axial displacement ha of the mandrel 1. As a result, the
axial stroke ha of the mandrel 1 and the radial travel hr of
the segments 6 of the tool 60 are in linear dependency
hr/ha=tgα, for each value of ha, respectively, the
transfer function between the axial ha and radial hr
displacements is linear. The safety valve 45 is set for the working
pressure.

[0042] After pressing the start button 36, located in handle 35 of the
device 61, voltage is applied from the control block 38 to the
electromagnet 41 of the control valve 43, and its adjacent section is
activated. As a result, the working fluid delivered by the hydraulic pump
46 passes through the right section of the control valve 43, the
non-return valve of the `throttle controllable--non-return valve` block
39 and through the pipe union 53 and hole 49 enters the piston chamber 50
of the hydraulic cylinder 27. The piston 26 starts running in the
direction of the front face 59 of the work piece 12 to be worked. The
working fluid pushed out of the rod chamber 51 passes through hole 52,
the pipe union 54, the controllable throttle of `throttle
controllable--non-return valve` block 40, the right section of the
control valve 43, filter 47 and enters the tank 48. The piston 26 by
means of the piston rod 25 moves axially the mandrel 1, which with its
working conical part 2 radially affects the segments 6. The latter move
radially to the surface of the hole 3 in the work piece 12 until the
pressure in the piston chamber reaches the preset value pmin. This
pressure guarantees that between the conical part 2 of the mandrel 1, the
segments 6 and the hole 3 to be worked, a hard contact without a
clearance is obtained.

[0043] After reaching the preset pressure pmin in the piston chamber
50, which is registered by the pressure sensor 37, a signal is sent to
the control block 38. The working stroke ha of the piston 26,
respectively the mandrel 1 takes place, such that the control block 38 by
the linear displacement sensor 34 monitors the reaching of the preset
value of the stroke ha. The mandrel 1 moves to the work piece 12 at
the specified distance ha, whereat the working surfaces 10 of the
segments 6 move radially to the hole 3 at distance hr, deforming it
plastically, whereat hr/ha=tgα. All points from the
working surface 10 of a given segment move parallel with its plane of
symmetry 17. The cold expansion of the hole to be worked is completed
after reaching stroke ha of the piston 26, respectively, the mandrel
1.

[0044] The formation of residual stress zone around the hole 3 in the work
piece 12 begins after switching the electromagnet 42 of the control valve
43 by the control block 38 As a result, the adjacent section of the
control valve 43 is activated and the working fluid from the pump 46
passes through the left section of the control valve 43, the non-return
valve of the `throttle controllable--non-return valve` block 40, the pipe
union 54 and through the hole 52 enters the rod chamber 51 of the
hydraulic cylinder 27. The piston 26, together with the piston rod 25 and
the mandrel 1 starts moving away from the work piece 12, whereat the
segments 6 owing to their elasticity and under the effect of the elastic
element 21 move radially to the axis 13, until their working surfaces 10
are separated from the hole 3 which is already expanded. The working
fluid pushed out of the piston chamber 50 of the hydraulic cylinder 27
passes through the controllable throttle of the `throttle
controllable--non-return valve` block 39, the left section of the control
valve 43, the filter 47 and enters the tank 48. After the piston 26 moves
away as much as possible from the work piece 12, i.e. takes an extreme
right position, the pressure rises, the safety valve 45 is activated and
the whole left section of the control valve 43 is shut down and thus the
formation of residual stress zone around hole 3 in work piece 12 is
completed and the operator releases button 36. The control block 38
allows the operator to release the start button 36 after finishing the
process of cold expansion and then the mandrel 1 is automatically
returned. After the effect of segments 6 on the hole 6 ceases, the
yielding layer of metal around the hole whose diameter has been increased
opposes the natural bias of the material particles of the work piece to
spring back to their initial position As a result of the blocked,
obstructed shrinkage of the metal, residual circumferential normal
compression stresses occur around the hole with minimum and symmetrical
axial gradient which, like a bracket close the existing microcracks and
impede the appearance of new ones.

[0045] Finally, the operator takes out the tool from the expanded hole 3
by means of the handle 35 of the device.

[0046] From the foregoing disclosure and detailed description of certain
preferred embodiments, it is also apparent that various modifications,
additions and other alternative embodiments are possible without
departing from the true scope and spirit of the present invention. The
embodiments discussed were chosen and described to provide the best
illustration of the principles of the present invention and its practical
application to thereby enable one of ordinary skill in the art to utilize
the invention in various embodiments and with various modifications as
are suited to the particular use contemplated. All such modifications and
variations are within the scope of the present invention as determined by
the appended claims when interpreted in accordance with the benefit to
which they are fairly, legally, and equitably entitled.

Patent applications by Yordan Todorov Maksimov, Gabrovo BG

Patent applications in class WITH DIRECT APPLICATION OF MAGNETIC FORCE TO MANIPULATE SHAPING MEANS OR WORK

Patent applications in all subclasses WITH DIRECT APPLICATION OF MAGNETIC FORCE TO MANIPULATE SHAPING MEANS OR WORK